BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

At the Less Than 1 Acre scale, it is typically unnecessary for engineeringcalculations to be performed to Size Conveyances. Therefore, no specificmodel is recommended for this application as shown in Table 1-1. If needed,the Rational Method could be used, but results may be sensitive to short timesof concentration when attempting to estimate peak flows at the Less Than 1Acre scale.INPUT DATA NEEDSThe following data inputs are necessary to estimate peak flow using theRational Method.• Runoff Coefficient (C) – this is a dimensionless factor used to convertrainfall intensity to runoff intensity. A number of hydrologic engineeringtextbooks provide recommended values based on land use type, soil type,and the distribution of impervious coverage.• Precipitation Intensity (i) – commonly estimated using NOAA14 depthduration-frequencycurves in the Lake Tahoe Region, where the recurrenceinterval and duration of the design storm are specified by the applicabledrainage design manuals for the project area. NOAA14 data for the LakeTahoe Region are accessible at: http://hdsc.nws.noaa.gov/hdsc/pfds/index.html• Drainage Area (A) – the area draining to an index point where an estimate ofpeak flow is desired (e.g. drainage area tributary to a culvert).• Time of Concentration (Tc) – travel time of runoff from the hydraulicallymost distant point of the drainage area to an index point. A commonmethod for estimating time of concentration is provided in TechnicalRelease 55 (TR-55), NRCS, 1986, available at:http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/Tools_Models/WinTR55.htmlTYPICAL LEVEL OF EFFORTRelative to the other models covered in these Summary Sheets, the RationalMethod is relatively easy to apply. Users should possess an intermediateunderstanding of hydrology to successfully apply the model. Most users wouldonly need a few hours of time to become comfortable applying the model.AVAILABILITY AND SUPPORTING DOCUMENTATIONThe Rational Method is a simple and commonly used equation for estimatingpeak runoff in small drainage areas with simple drainage conditions. A notablereference that provides the theory and application of the Rational Method is:Design and Construction of Urban Stormwater Management Systems. AmericanSociety of Civil Engineers and the Water Environment Federation. Manual ofPractice No. 77. New York: ASCE, 0-87262-855-8, 1993, 724 pp.CHAPTER 1: Urban HydrologyTRPA BMP HandbookPage 1-18 2012